This invention relates to current overload circuit protective devices and more particularly to overload relays utilizing electronic or so-called "static" trip units.
In the past, overload relays, such as utilized to protect electric motors against excessive current, have been of the thermal type wherein a bimetal actuator is heated directly or indirectly as a function of the motor current and operates to effect interruption of the motor circuit under overload conditions. More recently, overload relays have been designed to utilize electronic circuitry responsive to signals derived from the secondary windings of current transformers whose primary windings carry the motor phase currents. The electronic circuitry processes these signals on a current-time integral basis to determine when a current overload condition is sufficiently persistent to require interruption of the motor circuit. This static trip circuit protective approach has become quite popular because of its inherent versatility. That is, the electronic circuitry can be readily designed to recognize not only overload conditions, but also high fault current conditions calling for circuit interruption without delay and hazardous ground fault conditions. Moreover, the electronic circuitry can be readily implemented to provide for convenient calibration to a wide range of motor circuits of different current ratings and trip-time adjustability within a selected current rating to establish a highly repeatable trip-time curve precisely tailored to the particular motor to be protected from overcurrents ranging from light overload to heavy short circuit proportions. Thus, such static trip units may be manufactured in a standard design conducive to compact packaging as a modular unit adaptable to a wide range of circuit interrupter current ratings or motor frame sizes. Recently, it has been proposed to adapt such static trip unit to directly cause a motor starter to interrupt a motor circuit in the event of an overload condition, rather than to operate separate overload relay circuit interrupting contacts.
In adapting a standard static trip unit design to a particular motor circuit application, the trip unit must be calibrated to the current rating of the motor involved. Current calibration adjustability in static trip units is conventionally performed using either of two basic approaches (1) varying the tap settings on the current transform secondary windings, or (2) varying the value of the load resistance(s) across which signal voltages are developed in the input circuit section of the static trip unit. The latter and more popular approach typically involves utilizing rheostats, tap changing voltage divider switches, or rating plugs equipped with resistors of selected values which are plugged into the trip unit input circuit.
All of these prior art approaches to calibration adjustment involve closing of electrical contacts, the integrity of which can become degraded over time when subjected to the hostile environments often encountered in industrial applications. That is, the electrical contacts can and do become fouled with dirt, or loosen when subjected to repeated vibration or shock. This results in failure to hold calibration and thus proper overload protection is jeopardized. In addition, this degradation of electrical contacts can give rise to electrical noise which may cause trip unit malfunctions.
It is accordingly an object of the present invention to provide an improved static trip circuit protective device.
A further object is to provide a circuit protective device in the form of an overload relay which is readily adjustable over a range of calibration settings.
Another object is to provide an overload relay which is capable of reliably maintaining a calibration setting over a long operating life.
A still further object is to provide an overload relay which does not rely on continued integrity of electrical contacts in preserving its calibration setting.
An additional object is to provide a rugged overload relay which is efficient in construction, compact in size, and reliable over a long operating life, even in hostile environments.
Other objects of the invention will in part be obvious and in part appear hereinafter.